Synthetic oligonucleotides, their complexes and conjugates with other biomolecules represent valuable research tools and therapeutic agents. In spite of growing applications in basic research and clinical science, only few studies have addressed the issue of such compounds' stability in biological media. Herein, we studied the stability of two therapeutically relevant oligonucleotide probes in simulated biofluids; the 21 nucleotide-long DNA/locked nucleic acid oligonucleotide ON targeted toward cancer-associated BRAF V600E mutation, and a longer DNA analog (TTC) originating from BRAF gene. We found that stability of peptide–oligonucleotide conjugates (POCs) in human serum (HS) was superior compared with the naked or complexed 21mer oligonucleotide, whereas stability of POCs in simulated gastric juice (GJ) was dependent on the peptide sequence. Addition of pepstatin A in general increased the stability of oligonucleotides after 24 h digestion in HS and simulated GJ. Similarly, complexation with optimal amounts of histone proteins was found to rescue oligonucleotide stability after 24 h digestion in hydrochloric acid.